The invention relates to a strut for application between an outer ring and an inner ring in a gas turbine structure. The invention also relates to a gas turbine structure comprising an outer ring, an inner ring and a plurality of circumferentially spaced struts extending between the inner ring and the outer ring.
A gas turbine engine may be used as a jet engine. The term jet engine includes various types of engines, which admit air at relatively low velocity, heat it by combustion and shoot it out at a much higher velocity.
Accommodated within the term jet engine are, for example, turbojet engines and turbo-fan engines. The invention will below be described for a turbo-fan engine, but may of course also be used for other engine types.
An aircraft engine of the turbofan type generally comprises a forward fan and booster compressor, a middle core engine, and an aft low pressure power turbine. The core engine comprises a high pressure compressor, a combustor and a high pressure turbine in a serial relationship. The high pressure compressor and high pressure turbine of the core engine are interconnected by a high pressure shaft. The high-pressure compressor, turbine and shaft essentially form a high pressure rotor. The high-pressure compressor is rotatably driven to compress air entering the core engine to a relatively high pressure. This high pressure air is then mixed with fuel in the combustor and ignited to form a high energy gas stream. The gas stream flows aft and passes through the high-pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the high pressure compressor.
The gas stream leaving the high pressure turbine is expanded through a second or low pressure turbine. The low pressure turbine rotatably drives the fan and booster compressor via a low pressure shaft, all of which form the low pressure rotor. The low pressure shaft extends through the high pressure rotor. In civil applications most of the thrust produced is generated by the fan while in military applications most of the thrust produced is generated by the low and high pressure turbines. Engine frames are used to support and carry the bearings, which in turn, rotatably support the rotors. Conventional turbo fan engines have a fan frame, a mid-frame and an aft turbine frame. These frames constitute a gas turbine structure including an outer ring, an inner ring and a strut having a first end of the strut being attached to the ring and a second end being attached to the inner ring, the strut having first and second opposed strut faces being connected by a front edge and a rear edge.
In order to secure the gas turbine structure to a plane or another carrying structure which carries the gas turbine, the frame need to have mounting lugs to which links connecting the gas turbine to the plane or carrying structure must be present. Such mounting lugs are normally positioned in cup shaped indentations in an outer ring of the frame. The reason for arranging the mounting lug in an indentation of the outer ring is that forces transmitted in the plane of the outer ring should intersect at the mounting lug in order to eliminate that such forces generates a torque transfer via the lug to the link carrying the gas turbine. A strut is connected to each cup shaped indentation.
An example of a gas turbine structure is disclosed in U.S. Pat. No. 5,259,183. The indentations in the outer ring creates a disturbance to the gas flow through the turbine, which reduces the efficiency of the engine.
It is desirable to increase the efficiency of the gas turbine by reducing the disturbance on the gas flow through the turbine. The strut according to an aspect of the invention includes an end portion which has a diverging shape so that an internal space is formed in the end portion for receiving at least part of an engine mount. The invention, according to an aspect thereof, in particular relates to such struts at which engine mounting lugs are positioned.
In U.S. Pat. No. 5,259,183 the first and second opposed strut faces do not diverge in direction towards the ring. In U.S. Pat. No. 5,259,183 the strut is secured to a cup shaped indentation. The strut has a substantially uniform cross section along the radial extension of the strut. The strut is secured to the cup shaped indentation at the upper end of the strut. The joint between the strut and the indentation is a relatively sharp fillet radius. An area with a sharp fillet radius generates a local turbulent flow which has a negative impact on the disturbance on the gas flow through the turbine.
In contrast, a strut according to an aspect of the invention has an end portion of the strut with a diverging shape so that an internal space is formed in the end portion for receiving at least part of an engine mount. Due to the novel shape of the strut, the flow past the strut will be less disturbed. A particular reason for this is that the strut is free from fillet radius in between respective ends of the strut. Small radii of the outer surface of a strut, which radii are present in a flow channel where the struts may be mounted, will generate a distortion of the flow. The distortion of the flow may in turn lead to that an engine where the flow channel is included becomes less efficient. In particular small radii in an area that are somewhat distant to the ends of the struts at a connection between the strut and an inner and outer ring respectively will lead to flow distortion. Preferably, the diverging shape of the strut defines a smooth transition in direction from a center portion of the strut towards the strut end.
Hence the use of a strut according to an aspect of the invention in a flow channel will lead to reduced flow distortion in relation to prior art.
A strut may include first and second opposed strut faces being connected by a front edge and a rear edge. At the end portion of the strut, the strut faces are diverging in at least at central parts of the first and second opposed end faces. The strut faces are diverging in the direction toward an outer ring. By diverging the strut faces, at least at central parts of the opposed end faces, in the direction toward the outer ring, a smooth transition between the end faces and the connection of the strut to the outer ring can be accomplished. With central parts intended a central part of the strut with respect to an extension of the strut in a direction from a front edge to a rear edge of the strut. It may be sufficient to diverge the strut faces along this central part of the strut, which parts having a sufficient extension to make room for a recess adapted to accommodate component or a portion thereof such as typically an engine mount lug. It is therefore not necessary to diverge the strut faces at the front and rear edges of the strut where respective strut faces are connected. By arranging the strut faces to be diverging in the central parts, the strut is constructed with a geometry allowing an internal space, such as a recess, being arranged to receive a component or a portion thereof. The internal space is arranged at the end portion of the strut. The strut geometry thus has a reduced impact on the gas flow. This effect is achieved since the geometry of the strut according to an aspect of the invention to a lesser degree disturbs the gas flow through a flow channel where the strut may be mounted. For instance, the presence of a sharp fillet radius at a location in the vicinity of an inner end of a recess, which prior art solutions suffer from, is avoided.
The strut will have at least partially a curved shape. A cross section of the strut in a radial direction will thus have edge portion that diverge from each other in direction toward the end portion of the strut which houses the internal space. Each edge portion has the form of a curve which is diverging from a mid plane of the cross section. The curved shape may be concave or alternatively have both a concave and convex portion. By edge portion is intended an outer rim of the strut of a cross section through the strut.
The strut is provided with a smooth strut area. In the smooth strut area the curvature radius of edge portions of a cross section of the strut in a plane in the radial direction exceeds 0.15 L. Here L is the length of the strut. The length of the strut corresponds to the distance between an inner and an outer ring when the strut is mounted to connect the outer and inner ring. The invention, according to an aspect thereof, relates to a strut having a recess which is located in an end portion of the strut.
The recess is accessible externally of said outer ring in order to accommodate a component or part thereof. In particular the recess is arranged to receive a link for connection to a mounting lug present in the recess. The recess is having a depth D measured from a radius R of the outer ring, that is from an outer surface of the outer ring, to an inner end of said recess. The inner end of the recess is the location of the recess being closest to the inner ring. The distance is preferably measured in between two struts. In the event the outer ring and the inner ring have a conical shape, the depth D of the recess is measured at the location of the recess where the maximum distance in radial direction from the outer surface is present.
The smooth vane area is extending in the radial direction from a position located at a distance of D radially inside of the inner end to a position located at a distance of D/2 radially outside of said inner end.
Due to the requirement on the curvature imposed in the smooth vane area it is evident that no sharp fillet radius is present in the smooth strut area. The cross section may be taken along a length axis of the gas turbine structure or along a length extension of the strut that is a plane extending in the radial direction and being vertical to an axis extending from a front edge to a rear edge of the strut. The requirement imposed on the smooth vane area holds for both the cross section taken along the length axis of the gas turbine structure and the cross section taken across the length axis of the strut.
In an embodiment of the invention the edge portions are concave within said smooth strut area. This means that no inflection points at which the curvature radius change sign are present within said smooth strut area. The curvature radius may have its minimum value radially outside of said inner end of said recess and preferably within said smooth strut area. Furthermore a derivative of said curvature radius may be monotonously decreasing in a direction from the inner ring to the location of the minimum after which the curvature radius again may increase outwardly in a radial direction until the strut is connected with the outer ring. By forming the edge portions accordingly the smooth strut area is clearly void of any sharp structure which may have a negative impact on the gas flow through the turbine.
In another embodiment of the invention said edge portions changes from being concave to being convex within said smooth strut area. This enables the strut faces to more closely follow the form of the recess, while still fulfilling the requirement of keeping the smooth strut area void of any sharp structure. This enables reduction in consumption of material and may reduce the weight of the gas turbine structure. The curvature radius may have a minimum value in the smooth strut area at a location radially inside of said inner end of said recess. An inflection point at which said edge portions changes from being concave to being convex may be located within an inner end area extending from a location located D/6 radially inside said inner end of said recess to a location D/6 radially outside said inner end of said recess. The position of the inflection point at this location also enables the strut faces to closely follow the form of the recess. Furthermore a derivative of said curvature radius may be monotonously increasing outwardly in a radial direction in said smooth strut area up till said inflection point, at which the derivative will be decreasing. The curvature radius is thus decreased outwardly in the radial direction up to the location of the minimum value of the curvature from where the curvature radius increases up to the inflection point. After the inflection point, the curvature radius will be decreasing again. A second deflection point may be present in order to allow a smooth transition between the strut and the outer ring.
In order to further decrease the disturbance of the flow the smooth strut area may be increased in size to extend in the radial direction from a position located at a distance of 2D radially inside of said inner end to a position located at a distance of 4D/5 radially outside of said inner end. The smooth strut area may extend from the inner ring to the radius of the outer ring where the strut is connected to the outer ring.
The strut may be an integral structure including a recess formed at said first end of the strut, which recess is arranged to internally within the strut accommodate a component or a portion thereof.
In another embodiment the strut may be formed by connecting a lower strut portion to a top strut portion. The top strut portion is then constituted by a part containing the recess to accommodate the component. The division between the top strut portion and the lower strut portion may be formed at any location, but preferably radially inside of an inner end of the recess. The top strut part may be integral with the ring or a separate part connected to the ring. In the event a top strut portion including the recess is connected to a lower strut portion, regardless of whether the top strut portion is an integral part of the outer ring or a separate part connected to the ring, a smooth transition between the lower strut portion and separate body should preferably be present. Such smooth transition would fulfil the requirement of the strut faces stipulated above and the top strut portion would thus include portions of the strut faces referred to above. In this embodiment the strut has a length L which extends from the inner ring to the outer ring. For the purpose of this inventions the length L of the strut extends from the inner ring to the outer ring regardless of whether the strut is an integral piece or separate part or whether the strut is formed by connecting a lower strut portion with a top strut portion.
A joint between the strut and the ring may be positioned, in a radial direction, further from said inner ring than a bottom of said recess. By locating the joint between the strut and the outer ring as close to the radius of the outer ring as possible, that is a far from the inner ring as possible, the impact of any fillet radius present will be reduced. If the joint is located in the smooth strut area, the condition for the curvature radius imposed on the smooth strut area should be fulfilled. In the event the joint is located radially outside of the smooth strut area, the joint between the strut and outer ring may fulfil the condition for the curvature radius imposed on the smooth strut area. The outer ring would then form a continuous smooth surface at a sector including the location of the joint between the ring and the strut.
The invention, according to an aspect thereof, also relates to a gas turbine including strut as defined above.